Passive capacitive fuel probes come in two basic types: AC and DC. Both probe types are excited by a vehicle's fuel system (normally by a fuel gauge/conditioner unit) with an alternating current (AC) signal which typically has frequency in the range 1-10 kHz. The AC probe type produces a small AC output current. The DC probe type incorporates two diodes in a charge pump configuration and produces a half-wave rectified output current. The diodes introduce an error, which is measured and compensated for by the gauge/conditioner unit.
FIG. 1 is a schematic representation of a typical DC fuel probe. An AC excitation signal is routed to the low impedance (‘Lo Z’) and ‘Return’ terminals. The current output is measured at the high impedance (‘Hi Z’) terminal. Most gauges/conditioners have very low input impedance, in order to eliminate the effect of parasitic cable capacitance between the probe and the gauge.
It is reasonably straightforward to simulate an AC probe. There are a number of practical ways of producing a small AC current proportional to the desired capacitance. The situation is more difficult for DC probes as any variation between the electrical characteristics of the simulator and a real probe could have an unclear impact on the gauge's diode compensation algorithm.
It is known to use a number of selectable fixed-value precision capacitors (which can be selected using low capacitance relays) to create an electrically identical simulation of a probe. However, this approach requires costly specialised precision components and makes it difficult to achieve a high resolution.
Additionally, known simulation circuits normally only operate accurately with measuring circuits that have constrained operating characteristics (e.g. voltage/frequency/high impedance measurement impedance), as a result of the use in such simulation circuits of analogue to digital to analogue conversion.
Accordingly, a need exists for a method of accurately simulating an AC or DC fuel probe with high resolution and with a minimum of precision reference capacitors.